1. Field of the Invention
The invention relates generally to the field of computer networks and more specifically to a network comprising a plurality of stations which transmit data using a token-based protocol.
2. Description of the Prior Art
Computer networks enable various devices, such as processors, mass storage devices, and printers, to communicate with each other over high speed communication links. The devices, through interfaces, transmit and receive information in the form of frames transmitted over the communications links. The devices and their associated interfaces together are identified as stations. The stations operate in accordance with communication protocols which facilitate the orderly transfer of frames over the communication links.
One typical type of communication protocol is a token-ring system. In a token-ring system the stations are connected in a ring. Each station receives information frames over the communications link from one station and transmits frames over the communications link to another station in the network. If a station holds a token, it may originate a frame, transmitting it to another station in the network. Each of the other stations repeats the frame around the ring until it reaches the originating station. The station that is the intended recipient of the frame also copies the frame for processing.
The token is a coded information frame which is transferred around the network from station to station over the communications link. If a station needs to transfer a frame, when it receives the token from one station, it does not then transmit it to the next station in the network, effectively capturing the token. The capturing station then begins a frame transmission over the communications link. When the station is finished with the frame transmission, it transmits the token over the communications link to the next station, effectively releasing it. The stations thereafter transfer the token around the network until a station captures it.
The stations must follow predetermined network protocol rules for capturing, holding and releasing the token. One widely used token-ring protocol is the Fiber-Distributed Data Interface (FDDI) developed by the American National Standard Institute, Inc. The FDDI protocol, like many of the other token-ring protocols, establishes various time limits for capturing the token, holding it, and releasing it. For example, when a station captures the token it can hold the token, and thus transfer a frame, only for a predetermined amount of time. At the expiration of this holding time, the station must release the token, allowing the other stations on the network an opportunity to capture it. This ensures that no station monopolizes transmissions over the network. However, the station must release the token even if it has not yet completed transferring all of the information it has to transmit and even if no other station needs to perform a transfer. If no other station needs to perform a transfer, the released token will not be captured as it is being transferred around the network, and at the point the station again captures the token it may resume transmitting, but the network will have been idle while the token was being transferred.